1. Technical Field
The invention relates generally to imaging a large surface area of a semiconductor material, and it relates, in particular, to a method and apparatus for inspecting for crystallographic defects in the region near the surface of the material.
2. Prior Art
The production of integrated circuits, particularly those of VLSI size and complexity, requires that defects in the underlying semiconductor substrate be tightly controlled and monitored. The presence of a defect may cause a device fabricated on top of the defect to be either inoperative or only marginally operative. If the integrated circuit fabrication is carried through to completion, the presence of an inoperable device among the many thousands contained within an integrated circuit can be established through exhaustive and time-consuming testing. For a complete check, the testing must be performed at different values of operating temperature, power supply voltages and signal levels. For complex VLSI, a complete check on a finished device becomes prohibitively expensive.
The location of the fault or defect can be localized within a complex integrated circuit by scanning a completed circuit with a small beam of radiation. Lockhart et al. in U.S. Pat. No. 4,309,657 raster scan an electron beam over the integrated circuit. Ager et al. in U.K. Patent Application No. 2,069,152 perform a similar scan with a laser beam over an integrated circuit for which the supply voltage is reduced to a minimum level. While both of these methods are useful for pinpointing the location of a defect, they still require complicated electronic testing equipment and further require that the integrated circuit be completely fabricated. Furthermore, their methods, being electrical tests, do not provide information as to the type of defect causing the problem. It is desirable to have available a method which can detect the presence of defects in a semiconductor wafer before fabrication has begun or during fabrication, so as to eliminate the expense of processing defective wafers. It is further desirable that the defect detection process provide some information as to the type of defect so that a corresponding processing fault can be corrected.
A visual inspection of the entire wafer is possible but is very time-consuming and thus expensive if defects on the order of 1 .mu.m are to be detected. Steigmeier et al. in U.S. Pat. No. 4,314,763 disclose a method of wafer inspection in which a focused laser beam is scanned over the wafer. The intensity of the laser light reflected from the surface of the wafer is monitored and variations of the reflected intensity indicate the presence of defects at those locations on the wafer. While this method has the potential for fast and automated imaging of an entire wafer, it is in fact measuring the optical properties of the semiconductor material. Electrical defects, of overriding importance for integrated circuits, do not necessarily correspond to optical defects so that such a test method is incomplete.
In order to inspect and control silicon wafer quality, the current practice among integrated circuit manufacturers is to sample batches of wafers by means of a Zerbst technique. The sampled wafers are oxidized and then aluminum dot electrodes are evaporated onto the oxidized surface. Selected dots are probed electrically to determine the average minority carrier recombination rate. In practice, less than 1% of the surface area of the sampled wafer is actually probed in a typical test. Although this technique is useful in predicting device yield, this electrical test suffers many limitations. The test is destructive and cannot be used to characterize a specific wafer prior to device fabrication. The total area that is probed is only a small fraction of the total wafer area. Probing the entire surface would be very time consuming. Zerbst type measurements yield little or no information about the density or distribution of the defects on the surface. Finally, electrical probing is tedious and expensive. The wafer must be oxidized and then metallized with the aluminum dots. The individual dots must then be separately probed and analyzed.
Important electrical parameters which are affected by localized defects are the recombination rate, the minority carrier lifetime and the minority carrier diffusion length. These parameters are interrelated in well known semiconductor theory. Goodman, in U.S. Pat. No. 4,333,051, discloses a method of determining the minority carrier diffusion length in semiconductors by a measurement of the surface photovoltage in which an electrode is separated from an area of the semiconductor to be probed by an insulating layer and then light, shining through the electrode, produces a photo-generated charge which is detected across the electrode and a contact to the substrate of the semiconductor. However, the method of Goodman is too complex to easily provide an image of the entire wafer but is instead intended for a one-point measurement.
A similar technique is disclosed by Lile et al. in U.S. Pat. No. 4,051,437, in which a focused laser beam is scanned over the surface of a semiconductor material. One contact is made to the bulk semiconductor and another contact is made by either an MOS contact capacitively coupled into the semiconductor or by an aqueous electrolyte covering the surface through which the laser is directed. The laser beam generates a surface photo-voltage which is displayed in synchronism with the scanned laser beam to provide an image of the scanned semiconductor. This method provides a desirable type of display but appears capable of only limited resolution.
Philbrick and one of the present inventors have disclosed a device similar to that of Lile et al. in an article entitled "Scanned Surface Photovoltage Detection of Defects in Silicon Wafers" published in the 13th Annual Proceedings of Reliability Physics, pp. 159-167, 1975, in which the silicon is oxidized and a capacitively coupled electrode is placed over the oxide layer. Two-dimensional laser raster scanning produces large area imaging of the recombination centers at the surface of the semiconductor. Their method produces resolution of the order of 2 .mu.m.